This afternoon, we will be temporarily bringing down the CI, starting around 1 PM, California time, in order to upgrade to the latest Xcode beta and the associated SDKs. We will also be briefly locking master while we land some source changes in support of the updated SDKs on both master and the swift-4.0-branch. We expect the upgrade process to take an hour or two. We’ll send out a message when the upgrade is complete.
As a reminder, once the upgrade is complete, if you are doing development on a Mac, the minimum required Xcode to build Swift from source will be Xcode 9 beta 4. The beta can be downloaded at: https://developer.apple.com/download/